When switching inductive loads, such as in particular electrically recommutating the current flow of the winding(s) of EC motors, as a rule modern power semiconductor devices are used as semiconductor switches in combination with freewheeling elements, as a rule freewheeling diodes, which are parallel thereto. Power transistors such as MOSFETs or IGBTs are usual.
The semiconductor switch or switches is/are each controlled to switch or recommutate via their control connection using control signals, which are usually generated by a micro controller and then fed to a driver circuit, which then switches the respective semiconductor switch on or off.
In order to keep switching losses of power semiconductors to a minimum it is known to switch power conductors on and off as quickly as possible. Less power loss is achieved by this measure and cooling means can be kept accordingly small.
A disadvantage with switching inductive loads on and off rapidly is that, as a result of the steep edges when switching a power transistor, interference voltages are generated, which have to be suppressed again using so-called EMC filters.
The interference spectrum, the so-called EMC interference, forms as high-frequency interference when the power semiconductor is switched on and off. The limit value of such interference spectra are defined in certain EN standards and may not be exceeded. Therefore in many cases in practice it is necessary to construct costly, large-volume EMC filters, which require a lot of installation space and cause high costs. EMC filters are normally passive components, usually a combination of inductivity and capacity.